1. Field of the Invention
The present invention relates to a rubber composition comprising a crosslinked rubber particle component and a non-crosslinked rubber component as essential components. A rubber composition of the invention exhibits an excellent processability, and also exhibits excellent handling performance and dimension s and the like especially when combined with a silica as a reinforcing agent. A vulcanized rubber obtained by vulcanizing a rubber composition of the invention can be used in various industrial field as a rubber product such as a belt, a hose, a roll and a tire, and the like.
2. Description of the Related Art
A starting rubber which has been employed frequently for a rubber product may for example be:
(A) a polymer whose repeating unit consists only of a conjugated diene unit such as a butadiene rubber, an isoprene rubber, a chloroprene rubber and a natural rubber;
(B) a polymer whose repeating units are a conjugated diene unit and an aromatic vinyl unit such as a styrene-butadiene copolymeric rubber, a styrene-isoprene copolymeric rubber and a styrene-isoprene-butadiene copolymeric rubber;
(C) a polymer whose repeating us are a conjugated diene unit and an xcex1,xcex2-unsaturated nitrile unit such as an acrylonitrile-butadiene copolymeric rubber;
(D) a polymer in which an acrylate unit is essential as a repeating unit such as an acrylic rubber; and,
(E) a polymer whose repeating units are ethylene and an xcex1-olefin having 3 to 12 carbon atoms optionally with a non-conjugated polyene, such an ethylene-propylene rubber.
Any of these starting rubbers, when combined with a silica as a reinforcing agent, an provide a colored rubber product except for a black colored product, and a favorable performance of a formulated rubber made therefrom is a heat generation kept at a low level. However, unlike to a carbon black, a silica may lead to a problematic reduction in the tensile strength and the wearing resistance of a vulcanized rubber, and the like. Additional problems associated with the incorporation of a silica include {circle around (1)} a reduction in the processability and the compound consistency upon kneading, {circle around (2)} an insufficient smoothness of the surface of a sheet made from a compound using a roll and an irregular sheet edge accompanied with a substantial hrinkage upon cooling, and the like.
Such problems may be due an aggregation of a silica instead of a sufficient dispersion. A rubber composition in which a silica is not dispersed sufficiently poses a difficulty in obtaining a certain reinforcing effect of the incorporation of a reinforcing and undergoes a heat generation attributable to the interaction between poorly dispersed silica particles, resulting in a substantial reduction in the processability. Such poor dispersion of a silica occurs possibly because of the reasons that a silica has a high self-inter ting effect when compared with a carbon black and tends to aggregate easily and that a starting rubber usually consists of carbon and hydrogen and has a lower polarity when compared with a silica which leads to a low affinity with the silica.
For the purpose of increasing the affinity of a silica with a hydrocarbon-based rubber, the use of a conjugated diene-based rubber to which a functional group having an affinity with the silica is introduced has been investigated. For example, a conjugated diene-based rubber to which a hydroxyl group is introduced (WO96/23027), a conjugated diene-based rubber to which an alkoxysilyl group is introduced (JP-A 9-208623) and a conjugated diene-based rubber to which an alkoxysilyl group and an amino group or a hydroxyl group are introduced (JP-A 9-208633) have been proposed. However, such conjugated diene-based rubber to which a functional group described above is introduced mostly exhibits problematically poor dispersion, excessive heat generation upon processing and poor processability, and the like since it undergoes an extensive aggregation with a silica when the silica is admixed.
On the other hand, a silane coupling agent is incorporated usually for the purpose of preventing an aggregation of a silica and achieving a uniform dispersion. This results in an improvement in the dispersibility of a silica to some extent and a substantial improvement in the processability of a rubber composition and the tensile strength and the wearing resistance of a vulcanized rubber and the like. Nevertheless, the smoothness of the surface and the edge of a sheet or the problematic shrinkage of a sheet upon cooling or the like has not adequately been improved. Furthermore, a sufficient improvement in the processability, the tensile strength or the wearing strength or the like requires the incorporation of a silane coupling agent in an amount as high as about 8 to 15% by weight based on a silica, but such silane coupling agent is expensive and increases the cost for a rubber composition.
The present invention overcomes the problems experienced conventionally as described above, and is intended to provide a rubber composition having excellent processability and handling performance, which are experienced as a satisfactory processability upon kneading, a smooth sheet surface, a regular sheet edge and a less shrinkage upon cooling, and the like. A vulcanized rubber obtained by vulcanizing a rubber composition described above has excellent tensile strength and wearing resistance, and the like.
We made an effort to solve the problems mentioned above and discovered that by incorporating a crosslinked rubber particle component having a certain composition into a rubber composition, it is possible to obtain a rubber composition which exhibits a satisfactory processability upon kneading and achieves a satisfactory smoothness of the sheet surface and the sheet edge and a less sheet shrinkage upon cooling even when using a silica as a reinforcing agent. It was also discovered that by using this rubber composition it is possible to obtain a vulcanized rubber exhibiting a less heat generation and having sufficient tensile strength and wearing resistance, and the like. In addition, even when a silane coupling agent is used only in a reduced amount or is not used, the processability and the tensile strength, and the like were revealed to be improved sufficiently to the levels at which no practical problems are experienced.
According to the invention, a rubber composition having satisfactory processability and dimension stability, and the like can be obtained when using a silica as a reinforcing agent. In addition, a vulcanized rubber having excellent tensile strength and wearing resistance, and the like can also be obtained.